Inhibition of transmitter synthesis by a drug may happen in many ways. For example, a drug may inhibit the enzymes that neurons use to synthesize a particular neurotransmitter, resulting in depletion of that transmitter.

Drug inhibition of transmitter synthesis

Drugs that block axonal transport prevent materials from reaching the axon terminals, therefore causing the terminals to run out of neurotransmitters.

Drug working as a blockade of axonal transport

Drugs that do not prevent the production of transmitter molecules but instead store transmitters in synaptic vesicles for later release or they can prevent the packaging of transmitter molecules within vesicles and allow them to be broken down by enzymes. Both ways deplete transmitters at synapses.

Drug interfering with storage of transmitters

For a presynaptic neuron to produce neurotransmitters, the axon terminal must receive a steady supply of raw materials and the enzymes that use them to synthesize transmitter molecules, vesicles, and other important components. Some drugs alter this process by: 
- inhibition of transmitter synthesis
- blockade of azonal transport 
- interference with the storage of transmitters

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One way drugs can change synaptic transmission is by modifying the behavior of the presynaptic neuron, changing the system that converts an electrical signal (an action potential) into a chemical signal (secretion of neurotransmitter). Most common presynaptic drug effects are: 
- effects on transmitter production 
- effects on transmitter release 
- effects on transmitter clearance

Drugs altering presynaptic events

Breedlove, S. & Watson, N. (2017). Behavioral neuroscience, eighth edition. Oxford University Press.

Behavioral Neuroscience 8th Edition

Drugs affects on presynaptic transmitter production

Even with an adequate supply of transmitters in vesicles in the presynaptic terminal, various agents or conditions can prevent the release if transmitters when an action potential reaches the terminal. Examples of this include: 
- The prevention of synaptic transmission 
- Alteration of synaptic transmitter release
- Alteration of transmitter release through modulation of presynaptic activity 

Drugs effects presynaptically on transmitter release

Immediately after release, transmitter is rapidly cleared from the synapse. Getting rid of the used transmitter is normally an important step because unit it is gone, new release of transmitter from the presynaptic side won't be able to have much extra effect. 
 This can occur through: 
- The inactivation of transmitter reuptake
- The blockade of transmitter degradation

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